An electronic fuel injection system for engines such as automobile engines regulates fuel flow to the engine by varying the time interval that the injectors are held open through controlling the length of electrical pulses supplied to solenoid injectors. These injector opening periods are synchronized to the engine operating cycle by trigger pulses timed with the engine. The trigger pulses are typically generated by switching means such as reed switches in the distributor.
At high manifold pressures and rpm conditions, the injectors must be held open for the longest period to provide the required maximum fuel flow. To avoid degrading accuracy at low fuel requirement conditions, this period of maximum injector opening time is in the order of 10 milliseconds.
In high performance engines, engine speeds in the neighborhood of 8000 revolutions per minute are quite possible. Where injection trigger pulses are supplied alternately to two groups of cylinders, this corresponds to an injection pulse being called for every 7.5 milliseconds. Because the pulse generator is shared between the groups, it is not practical to generate overlapping pulse widths. If an injection trigger is received before the injection pulse has finished (i.e., 7.5 milliseconds pulse interval with 10 milliseconds pulse lengths), the pulse in progress is or may be immediately terminated, and the opposite group of injectors receive a trigger pulse. This will cause a leaning in fuel flow because the injectors are now open only 7.5 milliseconds instead of the 10 milliseconds really required for full performance.
A solution to this problem has been generated wherein, instead of injecting fuel in two groups, under some conditions of high-speed operation means are employed to inject all cylinders simultaneously. This makes it possible to supply trigger pulses with only half the frequency or once every 15 milliseconds. This permits a 10-millisecond required pulse width to be completed before the next trigger pulse arrives. Simultaneous injection arrangement is not satisfactory at lower engine operating speeds because of emission penalties, transient drivability problems and plug wetting due to injecting through open intake valves. Thus, the preferred method of operation is to provide the two-group injection described above with means for switching to simultaneous injection under certain high engine operating speed conditions. In U.S. Pat. No. 3,724,431 a system is described which switches from two-group operation to simultaneous operation, and vice versa, through the operation of a switch which simply responds to the sensing of a given engine operating speed. While quite operative, this system is believed to suffer from certain specific disadvantages. Since the pulse length of each injector pulse varies in accordance with engine manifold pressures, coolant temperatures, etc., the engine rpm at which the two-group injection pulses become of such length as to run into the timing problem described above is subject to considerable variation. Therefore, switching over to simultaneous injection at engine rotational speeds significantly lower than necessary can result in some emission penalties, transient drivability problems, etc., as described above. Another concern is that in operation in which the transition from two-group injection to simultaneous injection occurs simply as a result of operation of a single speed-responsive switch, there is danger of oscillation between the two modes at operating conditions close to the switching threshold.